Thanks guys, much appreciated.
Getting late here, so I'll have to pause for now.
If there's anything else you'd advise me to check or do just say and I can be getting on with it when I get started on it again.
Fair to say that a preamp with a 350mV DC offset reading shouldn't be awfully problematic?
Getting late here, so I'll have to pause for now.
If there's anything else you'd advise me to check or do just say and I can be getting on with it when I get started on it again.
Fair to say that a preamp with a 350mV DC offset reading shouldn't be awfully problematic?
The offset test points are for the main phono stage output, so he is not looking at the line stage output.
There are no test points for the line stage.
The rising and falling makes me think that it may be a cap charging and discharging..
Could it equally be a transistor related problem.. and if so wouldn't freeze spray be a good way of revealing which component was contributing to this effect?
Could it equally be a transistor related problem.. and if so wouldn't freeze spray be a good way of revealing which component was contributing to this effect?
It's almost certainly thermal drift, and not unusual in such a high DC gain phono circuit.
The input transistors will cause most of the drift, but it is DC coupled through the entire circuit.
The input transistors will cause most of the drift, but it is DC coupled through the entire circuit.
Sorry I was thinking cb1 was inadvertently monitoring the main output of the preamp (at the rca output sockets) and not that phono stage output (ie. TP1,2,3,4)
Yes, I suspect the adj pots can make the average DC offset (before the output coupling capacitor)
much larger than 200mV or 400mV. Probably several volts.
much larger than 200mV or 400mV. Probably several volts.
Probably not, in fact they may be specially selected for low noise, so
I would not try replacing them. All DC coupled circuits have thermal drift.
I would not try replacing them. All DC coupled circuits have thermal drift.
This entire thread concerns something of a non-issue. It is not right to call it offset as the preamp is entirely ac coupled. Whether the phono stage output is centered at zero or at 1v may have a tiny effect on the max output amplitude and distortion. Doubtful in the extreme if any measurements at normal operating levels can distinguish the difference.
Yes, he just needs to readjust the pots to meet the original spec, but it likely won't make a difference
in performance or sound. He was concerned there was a problem, which there is not.
in performance or sound. He was concerned there was a problem, which there is not.
Thanks everyone.
So, I should just adjust both channels so that with regard to this climbing and falling activity the midpoint is 0. Yes?
And be grateful that there isn’t actually an issue.
So, I should just adjust both channels so that with regard to this climbing and falling activity the midpoint is 0. Yes?
And be grateful that there isn’t actually an issue.